Generally, in printing machines for lithography, printing plates are wrapped around plate cylinders and mechanically fixed thereto, and in this state, printing is carried out.
However, lithographic printing plates using materials other than easily treatable metals (for example, plastic films and paper) as substrates have a disadvantage in dimensional stability. For example, they have the problem that the plates are partially strained by the friction between blanket cylinders and the plates, resulting in deterioration of the printing dimension and printing accuracy.
In the case of the plates in which the substrates as described are used, therefore, the use thereof has hitherto been limited to easy methods in which the register accuracy of printed matter is not required and a small number of sheets are printed, and they have not been used as such in high-grade multicolor accurate printing and full-scale printing using large-sized printing machines.
In order to solve such a problem, JP-A-63-109090 proposes a method of directly adhering a plate material to a plate cylinder or a block copy plate for adjusting touch pressure between a printing plate and a blanket cylinder by use of, for example, an acrylic or rubber adhesive double coated sheet or spray adhesive.
In such a method, however, the problem newly arises that fine adjustment of the position of the printing plate on the plate cylinder is impossible, or that the workability of plate discharge and the like after printing is extremely deteriorated, because the printing plate is firmly adhered to the plate cylinder.
Further, JP-B-7-425 proposes a method of providing a plate cylinder of a printing machine with a specified silicone rubber film to improve the printing accuracy.
According to this method, however, the silicone rubber film is not separated from the printing machine, so that, for example, it is difficult to simply remove it as the conventional block copy materials for the adjustment, and when printing plates different in thickness are used in the same printing machine, it becomes necessary to replace a block copy material for the adjustment to the plate thickness thereof. This method therefore has the problem that the workability of this replacement is extremely deteriorated.